Initial compounds for producing polyurethanes

ABSTRACT

The invention relates to starting compounds which can be used for the preparation of polyurethanes and can be prepared by reaction of hydroxyl-containing oligomers of formaldehyde.

The present invention relates to novel starting compounds for thepreparation of polyurethanes and to a process for preparing them.

Polyurethanes and their preparation have been known for a long time andhave been described many times in the literature. They are usuallyprepared by reacting polyisocyanates with compounds having at least twohydrogen atoms which are reactive toward isocyanate groups.

As compounds having at least two hydrogen atoms which are reactivetoward isocyanate groups, use is usually made of polyols. Among these,polyether alcohols and polyester alcohols have the greatest industrialimportance.

Polyester alcohols are usually prepared by reacting at leastbifunctional alcohols with at least bifunctional carboxylic acids.

The polyether alcohols are generally obtained by addition of alkyleneoxides onto OH- or NH-functional starter compounds.

The price of the polyols customary hitherto is determined by the startercompounds used and the alkylene oxides employed, in particular propyleneoxide and ethylene oxide.

A significantly cheaper starting compound for preparing polyols would beformaldehyde. It is known that formaldehyde reacts with itself to formoligomers having terminal hydroxyl groups. However, this reactionusually leads to a mixture of oligomers and polymers of varying chainlength which are in equilibrium with formaldehyde. Compounds having sucha broad molecular weight distribution cannot be used for preparingpolyurethanes. A further disadvantage of these compounds is theirunsatisfactory stability. Even after a short time, redissociation of theoligomers and polymers occurs.

DD 247 223 describes a process for preparing polyether alcohols in whicha mixture of formaldehyde condensates, known as formose, and othercompounds having active hydrogen atoms is reacted with alkylene oxides.In this process, too, the formose has a broad molecular weightdistribution.

EP 1 063 221 describes a method for preparing formaldehyde oligomers ofthe formula (I) having a narrow molecular weight distribution fromformaldehyde. The reaction proceeds according to the equation

Here, formaldehyde oligomers of the formula (I) in which n=2 to 19,preferably 2-9, can be obtained by dewatering of formaldehyde solutions.These solutions can contain up to 80% of oligoformaldehyde, and freewater is not present. It is possible to separate off individualfractions, i.e. oligomers having particular chain lengths, by means of aparticular procedure, especially by means of distillation. The oligomersare reacted with other substances, exploiting the defined redissociationof the oligomers to formaldehyde.

The resulting oligomer mixture comprising generally 2-9 formaldehydeunits, including polyoxymethylene, is unstable. Within 2 hours, highermolecular weight compounds (paraformaldehyde), water and monomolecularhydrated formaldehyde are formed.

It is an object of the present invention to reduce the costs of thestarting materials for the preparation of polyurethanes, in particularthe costs of the polyols.

We have found that this object is achieved by using formaldehyde and itsoligomers in a simple fashion as starting substance for the preparationof polyurethane raw materials.

The present invention accordingly provides starting compounds for thepreparation of polyurethanes, hereinafter also referred to aspolyurethane raw materials, which can be prepared by reaction ofhydroxyl-containing oligomers of formaldehyde.

The invention further provides a process for preparing polyurethane rawmaterials by reaction of the hydroxyl groups of oligomers offormaldehyde.

As oligomers of formaldehyde, use is made of mixtures of compounds ofthe formula (I),

where n is an integer from 2 to 19, in particular from 2 to 9.

The compounds of the formula (I) can be prepared by known methods. Thus,the oligomers can be produced by the known polymerization of trioxane, acyclic reaction product of formaldehyde. This process is known from theliterature. It is preferably employed for preparing polyoxymethylene(POM) and is described, for example, in Römpp Chemie Lexikon. However,this process is not preferred for the preparation of the polyurethaneraw materials of the present invention, since high molecular weightreaction products are preferentially formed.

In a preferred embodiment of the invention, the oligomers are preparedby the process described in EP 1 063 221, with the subsequent reactionof the oligomers with aniline described in this document being omitted.

The preparation of the oligomers of formaldehyde is carried out byseparating off particular fractions from a solution in whichformaldehyde and its oligomers are in equilibrium. This separation ispreferably carried out by distillation, usually by means of a filmevaporator, in particular by means of a thin film evaporator. Suitableoperating conditions for the film evaporator are generally a temperatureof from 10 to 230° C., preferably from 10 to 150° C., and an absolutepressure of from 0.5 mbar to 2 bar. Temperatures of from 20 to 100° C.and atmospheric pressure are preferred for the fractionation of anaqueous formaldehyde solution.

The fractions of oligomers of formaldehyde which have been separated offin this way usually have a very narrow molecular weight distribution.They are, as mentioned above, storage-stable for a particular time andshould be processed further during this time in order to avoid a changein their composition.

In principle, separation of particular oligomers from the reactionmixture could be omitted and the mixture be used directly for preparingthe starting compounds for the preparation of polyurethanes. However,this has the disadvantage that this mixture contains large amounts offree formaldehyde and water, resulting in a high level of secondaryreactions.

The above-described oligomers of the formula (I) can in principle alsobe used without further treatment as starting compounds for thepreparation of polyurethanes. If the reaction immediately follows theirisolation, degradation reactions which lead to elimination offormaldehyde are avoided. However, preference is given to carrying out areaction of the terminal hydroxyl groups of the oligomers.

In the reaction of the oligomers to form starting compounds for thepreparation of polyurethanes, their terminal hydroxyl groups arereacted.

In one embodiment of the invention, the terminal hydroxyl groups arereacted with alkylene oxides to form polyether alcohols. The reaction isusually carried out as in the known preparation of polyether alcoholsusing the customary alcoholic starter substances.

The hydroxyl number of the oligomers is, depending on the number offormaldehyde units, in the range from 1 436 mg KOH/g at n=2 to 389 mgKOH/g at n=9. Since the oligomers are water-free after they have beenseparated off, a drying step between isolation of the oligomers andtheir reaction with the alkylene oxides is no longer necessary.

As is customary in industry, the reaction of the oligomers offormaldehyde with the alkylene oxides is carried out in the presence ofcatalysts. Catalysts which can be used are, as is customary, basiccompounds such as amines, basic metal oxides and metal hydroxides, inparticular potassium hydroxide.

Preferred catalysts are multimetal cyanide compounds, also referred toas DMC catalysts. Such compounds have been known for a long time and aredescribed, for example, in EP 654 302 or EP 862 947. Advantages of theuse of DMC catalysts are firstly that they can remain in the productafter the reaction and secondly that, in contrast to basic catalysts,they do not promote redissociation of the oligomers.

As alkylene oxides, it is possible to use the compounds which are knownand customary for this purpose. Ethylene oxide and propylene oxide,which can be used individually or in any mixtures with one another, havethe greatest industrial importance. When ethylene oxide and propyleneoxide are used, the two alkylene oxides can be introduced together toproduce a random polyether chain or in succession to form alkylene oxideblocks.

The type and amount of alkylene oxides introduced depends, inparticular, on the use to which the polyether alcohols are to be put.For use in rigid foams, the polyether alcohols have short chains. Thehydroxyl number of such polyether alcohols is usually in the range from300 to 600 mg KOH/g, in particular from 400 to 500 mg KOH/g. As alkyleneoxide, preference is given to using propylene oxide.

For use in flexible foams, use is usually made of long-chain polyetheralcohols. The hydroxyl number of these polyether alcohols is usually inthe range from 30 to 120 mg KOH/g, preferably in the range from 30 to 60mg KOH/g. As alkylene oxides, use is usually made of mixtures ofethylene oxide and propylene oxide. In the case of particularapplications, for example for the production of cold-cure molded foams,a pure ethylene oxide block is added on at the end of the polyetherchain.

When DMC catalysts are employed for preparing the polyether alcohols,preference is given to using propylene oxide or a random mixture ofpropylene oxide and ethylene oxide as alkylene oxide. In a preferredembodiment of this process, a random mixture of ethylene oxide andpropylene oxide is metered in and the ratio of the two alkylene oxidesin the mixture is altered during the metered addition, as described inWO 01/44347.

This process variant makes it possible to prepare diols having a narrowmolar mass distribution within wide molar mass ranges in a simplefashion by use of oligomers of different molecular weights.

The oligomers of formaldehyde can be reacted either alone or inadmixture with other H-functional starter substances with the alkyleneoxides. As additional starter substances, preference is given to usingat least bifunctional alcohols such as glycerol, trimethylolpropane,ethylene glycol, propylene glycol or their higher homologues.

The reaction of the starter substance with the alkylene oxides isgenerally carried out at the pressures customary for this purpose in therange from 0.1 to 1.0 MPa and the customary temperatures in the rangefrom 80 to 140° C. The introduction of the alkylene oxides is usuallyfollowed by an after-reaction phase to complete the reaction of thealkylene oxides. In an advantageous embodiment of the process of thepresent invention, a further catalyst, in particular amine catalyst, isadded to the reaction mixture at the beginning of the after-reactionphase, preferably immediately after the introduction of the alkyleneoxides has been concluded.

After the addition reaction of the alkylene oxides, the polyetheralcohols are usually subjected to a brief treatment by distillation toseparate off volatile impurities. If necessary, the polyether alcoholcan subsequently be filtered to remove any solid impurities present. Ifbasic compounds are used as catalysts, the catalyst is removed after theaddition reaction of the alkylene oxides. This can be achieved byneutralization with acids or by use of adsorbents. The salts or theadsorbents are subsequently removed by filtration.

In a particular embodiment of the process of the present invention, thereaction of the oligomers of formaldehyde with the alkylene oxides, inparticular when using DMC catalysts, can also be carried outcontinuously. In this case, the oligomer mixture which has beenseparated off and the alkylene oxide and also the catalyst areintroduced continuously into a reactor and the polyether alcohol formedis taken off continuously from the reactor. Such continuous processesare described, for example, in DD 203 235 and WO 98/03571. Thecontinuous reaction can be carried out, for example, in tube reactors,stirred vessels or loop reactors. In this variant of the process of thepresent invention, the continuous preparation of the polyether alcoholsby reaction of the oligomers with alkylene oxides can immediately followthe likewise continuous isolation of the oligomers.

The polyether alcohols obtained in this way can be reacted withisocyanates without problems by customary methods to give polyurethanes.Here, the polyether alcohols of the present invention can be used eitheralone or preferably in admixture with other compounds such as additionalalcohols, in particular short-chain polyfunctional alcohols, polyetheralcohols and/or polyester alcohols, preferably polyether alcohols. Asshort-chain alcohols, use is usually made of bifunctional orpolyfunctional alcohols having a molecular weight in the range from 62to 400 g/mol, e.g. ethylene glycol, propylene glycol and their higherhomologues or glycerol.

As polyether alcohols and polyester alcohols, it is possible to use thecompounds which are known and customary for this application. Theyusually have a molecular weight M_(n) of above 400 g/mol, preferably inthe range from 400 to 15 000 g/mol. These polyols are prepared bycustomary and known methods, in the case of the polyester alcohols byreaction of polyfunctional alcohols with polyfunctional carboxylicacids, and in the case of the polyether alcohols by addition of alkyleneoxides onto H-functional starter substances. The reaction may, dependingon the type of polyurethanes desired, be carried out in the presence ofcatalysts, blowing agents and customary auxiliaries and/or additives.

In a further embodiment of the present invention, the oligomers offormaldehyde are, after they have been separated off from the reactionmixture, reacted with isocyanates to form prepolymers.

For this purpose, the terminal hydroxyl groups of the oligomers of theformula (I) which have been separated off as described above are reactedwith isocyanates. Since the oligomers are storage-stable for only alimited time, the reaction in this case, too, has to be carried outimmediately after isolation of the oligomers if a product having anarrow molar mass distribution is to be obtained.

If the oligomers are stored for too long, not only does the molar massdistribution become broader but formation of formaldehyde and water inthe oligomer mixture also occurs. Although the formaldehyde formed canin principle be removed by stripping, its formation is disadvantageousfor further processing of the oligomers to form prepolymers, since itleads to undesirable secondary reactions.

As a result of the reaction of all hydroxyl groups of the oligomers,redissociation of the oligomers is completely suppressed. Theprepolymers are storage-stable and can be processed like prepolymersderived from other polyols customary in polyurethane chemistry.

The reaction of the hydroxyl-containing oligomers with the isocyanatesis carried out in the manner customary for preparing prepolymerscontaining isocyanate groups. The oligomer is for this purpose reactedwith at least that amount of isocyanate which suffices for completereaction of the hydroxyl groups of the oligomer. The reaction can becarried out in the presence of customary urethane formation catalysts.To carry out the reaction, it is usual to place the isocyanate compound,if desired in the presence of a catalyst, in a reaction vessel at from40 to 100° C., preferably from 50 to 80° C. The oligomer mixture ismetered in while stirring, and the reaction mixture is subsequentlyallowed to react further at from 60 to 140° C., preferably from 80 to100° C., usually for up to two hours, if appropriate to completeconversion.

The NCO content of the prepolymers is dependent on the molar mass of theoligomers, the excess of isocyanate used, the reaction time, theresidence time, the reaction temperature and the catalysts used.

The NCO content of the prepolymers is usually in the range from 10 to30% by weight, preferably from 15 to 25% by weight.

The oligomers of formaldehyde can be reacted either individually or inadmixture with other compounds having at least two hydrogen atoms whichare reactive toward isocyanate groups with the isocyanates. Thecomponents which can be reacted together with the oligomers offormaldehyde with isocyanates to form prepolymers are, in particular,alcohols. Depending on the intended use of the prepolymers, it ispossible to use various alcohols in an amount of from 0 to 90% byweight, preferably from 0 to 60% by weight, in each case based on thesum of the oligomers of formaldehyde and the other compounds having atleast two hydrogen atoms which are reactive toward isocyanate groups.

For most applications, the additional alcohols used are short-chainpolyfunctional alcohols, polyether alcohols and/or polyester alcohols,preferably polyether alcohols. As short-chain alcohols, it is usual touse bifunctional or polyfunctional alcohols having a molecular weight inthe range from 62 to 400 g/mol, e.g. ethylene glycol, propylene glycoland their higher homologues or glycerol.

As polyether alcohols and polyester alcohols, it is possible to use thealcohols which are customary for this application and have beendescribed in more detail above. It is also possible for the polyetheralcohols which have been prepared by addition of alkylene oxides ontooligomers of the formula (I) to be reacted together with the oligomersof formaldehyde with isocyanates.

Isocyanates which can be used in the process of the present inventionare all isocyanates having two or more isocyanate groups in themolecule. It is possible to use both aliphatic isocyanates such ashexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI) orpreferably aromatic isocyanates such as tolylene diisocyanate (TDI),diphenylmethane diisocyanate (MDI) or mixtures of diphenylmethanediisocyanate and polymethylenepolyphenylene polyisocyanates (P-MDI),preferably TDI and MDI. It is also possible to use isocyanates whichhave been modified by incorporation of uretdione, isocyanurate,allophanate, uretonimine and other groups. These compounds arefrequently also referred to as modified isocyanates.

The prepolymers which have been prepared in this way can be reacted withcompounds having at least one, preferably at least two, hydrogen atom(s)which is/are reactive toward isocyanate groups in the molecule to givepolyurethanes. Depending on the type of polyols and isocyanates used,the prepolymers can be processed to produce rigid foams, flexible foams,adhesives, coatings or elastomers.

The invention is illustrated by the following examples.

EXAMPLE 1

A formalin solution having a formaldehyde content of 37% by weight wasevaporated to a theoretical formaldehyde content of 73% by weight bymeans of a thin film evaporator at a wall temperature of 80° C. and 120mbar. The solution was stored at 80° C. and processed further within onehour. 961 g of this solution were admixed with 38.4 g ofdimethylcyclohexylamine in a pilot plant autoclave and 1 010 g ofpropylene oxide were metered in at 100° C. over a period of 6 hours. Thereaction mixture was subsequently allowed to react further at the sametemperature for 2 hours. Volatile constituents were then removed underreduced pressure. The liquid reaction product which remained had ahydroxyl number of 685 mg KOH/g and a water content of 0.011% by weight.Analysis by GPC indicated oligomeric products having a molar mass in therange 100-500 g/mol. Examination by means of gas chromatography and massspectroscopy coupled thereto (GC-MS) showed that adducts of twomolecules of propylene oxide and two molecules of formaldehyde had beenformed.

EXAMPLE 2

A formalin solution having a formaldehyde content of 37% by weight wasevaporated to a theoretical formaldehyde content of 73% by weight bymeans of a thin film evaporator at a wall temperature of 80° C. and 120mbar. The solution was stored at 80° C. and processed further within onehour. 1 110 g of this solution were admixed with 70 g of potassiumhydroxide in a pilot plant autoclave and 1 600 g of propylene oxide weremetered in over a period of 9 hours. The liquid reaction product had ahydroxyl number of 868 mg KOH/g and a water content of 0.014% by weight.Analysis by means of gel permeation chromatography indicated oligomericproducts having a molar mass in the range 100-500 g/mol. GC-MS showedthat adducts of two molecules of propylene oxide and two molecules offormaldehyde had been formed.

1. A starting compound which can be used for preparing polyurethanes andcan be prepared by reaction of hydroxyl-containing oligomers offormaldehyde.
 2. A compound as claimed in claim 1, wherein oligomers offormaldehyde having the formula (I),

where n is an integer from 2 to 19, are used.
 3. A compound as claimedin claim 1 which can be prepared by reaction of the hydroxyl groups ofthe oligomers of formaldehyde with alkylene oxides.
 4. A compound asclaimed in claim 3, wherein ethylene oxide, propylene oxide or a mixtureof ethylene oxide and propylene oxide is used as alkylene oxide.
 5. Acompound as claimed in claim 1 which is prepared by reaction of hydroxylgroups of the oligomers of formaldehyde with isocyanates.
 6. A compoundas claimed in claim 5 which has an NCO content in the range from 10 to30% by weight.
 7. A compound as claimed in claim 5 which has an NCOcontent in the range from 15 to 25% by weight.
 8. A process forpreparing compounds as claimed in claim 1 by reaction of the hydroxyl-containing oligomers of formaldehyde.
 9. A process as claimed in claim8, which comprises the steps a) preparation of oligomers of formaldehydehaving the formula (I), b) reaction of the hydroxyl groups of theoligomers of formaldehyde.
 10. A process as claimed in claim 9, whereinthe oligomers are separated off from an aqueous formaldehyde solution bydistillation in step a).
 11. A process as claimed in claim 10, whereinthe distillation is a thin film distillation.
 12. A process as claimedin claim 9, wherein a reaction with alkylene oxides is carried out instep b).
 13. A process as claimed in claim 9, wherein a reaction withisocyanates is carried out in step b).
 14. A process as claimed in claim9, wherein the steps a) and b) are carried out continuously.
 15. Aprocess for preparing polyurethanes, which comprises reacting compoundsas claimed in claim 3 with isocyanates.
 16. A process for preparingpolyurethanes, which comprises reacting compounds as claimed in claim 5with compounds having at least two groups which are reactive towardisocyanates.